1. Field of the Invention
The present invention relates to optical systems operative to filter out high numerical aperture (“NA”) rays from laser light propagating through free space.
2. Description of the Related Art
To meet current industrial requirements, lasers in general including solid and fiber lasers become more and more powerful. High light powers however often should be attained without degrading the quality of laser light. Propagation of laser light through free space in various configurations of fiber laser systems is customary. For example, pigtailed optical components of fiber laser systems, such as isolators, circulators and the like, are configured with light path stretches requiring light to propagate via free space. Still another optical configuration requiring light propagation via free space includes laser diode modules typically operating as pumps sources. A laser diode module typically has a plurality of laser diodes which emit respective beams. The beams are further collimated and focused on the entry faucet of a multimode (“MM”) delivery fiber which further guides pump light to fiber gain blocks.
The coupling of the focused light into an optical fiber is of importance in general and, in particular, when a laser diode module functions as pump for fiber laser systems capable of emitting laser light. The progress observed in various industries, as a rule, requires increasingly high laser beam powers with a high quality of the laser beam.
Meeting this requirement is so-called high power fiber laser systems typically including one or multiple diode pump modules. The generated pump light is then coupled into one or multiple gain blocks. A gain block is configured with an active fiber typically having a multimode (“MM”) core which is structured to support substantially only a fundamental mode (“FM”) at the desired wavelength. Input and output single mode (“SM”) passive fibers, coupled to respective opposite ends of the active fiber, typically complete a fiber unit of gain block.
A pump laser diode module for a high power laser system is configured as a pigtailed optical element, i.e., the element with an optical fiber coupled thereto. Accordingly, focused MM light from a plurality of emitters is coupled into a MM passive fiber which, true to its definition, supports multiple transverse modes including the most powerful, fundamental mode. The modes “compete” for a given power. With a higher number of HOMs excited in the MM passive fiber, the power of the fundamental mode lowers.
The power loss of the delivered fundamental mode greatly affects the overall efficiency of the high power fiber laser system disclosed above. Accordingly, it is desirable that the fundamental mode delivered by the MM passive fiber be maximally powerful which can be achieved by reducing the number of HOMs.
There are several factors influencing the number of excited HOMs. For example, during the coupling process of the focused light into MM passive fiber, most of the light enters into the MM core. Such light predominantly excites central core modes with the intensity of the light power aligned around the core region of the optical fiber which is mainly occupied by a fundamental mode and a few central HOMs.
But some laser light rays with respective large numerical apertures, i.e., rays that propagate at relatively great angles with respect to the optical axis of the fiber, stray away from and are not well aligned with the MM core. Such stray rays may excite a great deal of peripheral core HOM typically propagating along the core/cladding interface and cladding modes of the fiber. These peripheral HOMs also reduce the power and quality of the fundamental mode and, in addition, quite often tend to couple out of the fiber presenting environmental hazard. In particular, a protective polymeric layer shielding a fiber from mechanical stresses, may be easily damage which often leads to irreparable damages to the fiber itself. In addition, decoupled light is hazardous to other optical components of a fiber system.
Conventionally an aperture stop or a diaphragm is used to filter away the strayed laser rays from entering into the fiber. This straightforward solution generally works well for regular free space optics. However, in micro-optics associated with high power pump modules, the aperture stop is more difficult to handle. Therefore, the requirement for high precision in mounting and alignment with respect to the optical fiber axis cannot always be satisfied. The latter, in turn, decreases reliability and reproducibility of laser modules for high volume production.
The excitement of peripheral modes causing the above-discussed undesirable consequences is not exclusively limited to a pump light delivery system. Fiber laser systems often have stretches of light path along which collimated light propagates through free space and then is coupled into a fiber. For example, a termination block has collimating lenses or multi-cascaded high power fiber laser system often includes isolators, circulators and other bulk components configured with free space path stretches before the light is to be coupled into a fiber. In many instances, the coupling of light propagating through into a fiber may have the same problems as disclosed above in regard to a pump light delivery system.
A need, therefore, exists for an improved method of minimizing excitation of peripheral and cladding HOMS in a passive MM fiber receiving MM light travelling via free space.
A further need exists for an optical system implementing the improved method.